Resumen del trabajo presentado al Dynamics of Soft Matter Workshop, celebrado en Boston (USA) del... more Resumen del trabajo presentado al Dynamics of Soft Matter Workshop, celebrado en Boston (USA) del 4 al 6 de diciembre de 2008.We have exploited the selectivity of neutron scattering combined with isotopic substitution to study the structure and dynamics of poly(n-alkyl methacrylates). Our diffraction data strongly support the suggested nanosegregation of main-chains and side-groups. Moreover, we have been able to separately follow the dynamics of both subsystems at a molecular scale. While the structural relaxation observed at the main-chain level is standard, for high-order members the correlations involving side groups within the alkyl nanodomains relax through an exotic logarithmic decay. In these polymers, a strong dynamic asymmetry also develops. We can find similarities with other soft materials where nanoconfinement is a key ingredient, like dynamically asymmetric miscible polymer blends. These analogies suggest a general scenario for the anomalous relaxation situation observed which could be explained in terms of a high-order Mode Coupling transition.Peer reviewe
A classical molecular dynamics study of the diffusive motion of benzene molecules on graphite bas... more A classical molecular dynamics study of the diffusive motion of benzene molecules on graphite basal planes has been performed in the sub-monolayer coverage regime. Atomistic calculations were performed using the second generation forcefield COMPASS as well as the general purpose forcefields Dreiding and Universal. The COMPASS based calculations show evidence for a Brownian nature of the diffusion with a very small diffusion activation barrier of 11±2meV in agreement with recent helium and neutron spin-echo spectroscopy data [Fouquet P, Hedgeland H, Jardine AP, Alexandrowicz G, Allison W, Ellis J. Measurements of molecule diffusion on surfaces using neutron and helium spin echo. Physica B 2006;385–386:269-71]. Reasonable agreement is also found for the general purpose forcefields if screened charges are used in the description of the Coulombic non-bond interaction. The less computationally intensive Dreiding forcefield was shown to give a good qualitative description of the diffusion validating its applicability for future large scale calculation, i.e., for long times or enlarged systems. A potential energy surface (PES) has been established for the translational and rotational diffusive motion. The PES shows a peculiar dependence of the lateral diffusion barriers on the rotation angle of the benzene molecule which leads to a preferential selection of certain rotational states in the MD trajectories.
HAL (Le Centre pour la Communication Scientifique Directe), 2008
International audienceQuasi-elastic neutron scattering (QENS) measurements have been performed to... more International audienceQuasi-elastic neutron scattering (QENS) measurements have been performed to investigate the surface selfdiffusion of hydrogen molecules. A monolayer of molecular hydrogen was adsorbed on a carbon material commonly used in polymer electrolyte membrane fuel cells, called XC-72. QENS spectra were recorded at the time-of-flight spectrometer IN5 at Institut Laue-Langevin (ILL) in Grenoble at 40, 50, 60, and 70 K. By using the Chudley & Elliott model for jump diffusion, we found the diffusion coefficient at each temperature. The logarithm of the diffusion coefficient was plotted versus the inverse of the temperature to give the coefficient in the Arrhenius equation. From this, we can estimate the diffusion at higher temperatures as well. Our observed diffusion follows the same trend as that found for hydrogen molecules on the surface of singlewalled carbon nanotubes and Grafoil
The binding and structure of an ordered pyrazine overlayer on graphite are studied with neutron s... more The binding and structure of an ordered pyrazine overlayer on graphite are studied with neutron scattering and DFT. We find increased van der Waals bonding and stability of the overlayer compared to benzene, due to the nitrogen atoms in the ring.
Resumen del trabajo presentado al Dynamics of Soft Matter Workshop, celebrado en Boston (USA) del... more Resumen del trabajo presentado al Dynamics of Soft Matter Workshop, celebrado en Boston (USA) del 4 al 6 de diciembre de 2008.We have exploited the selectivity of neutron scattering combined with isotopic substitution to study the structure and dynamics of poly(n-alkyl methacrylates). Our diffraction data strongly support the suggested nanosegregation of main-chains and side-groups. Moreover, we have been able to separately follow the dynamics of both subsystems at a molecular scale. While the structural relaxation observed at the main-chain level is standard, for high-order members the correlations involving side groups within the alkyl nanodomains relax through an exotic logarithmic decay. In these polymers, a strong dynamic asymmetry also develops. We can find similarities with other soft materials where nanoconfinement is a key ingredient, like dynamically asymmetric miscible polymer blends. These analogies suggest a general scenario for the anomalous relaxation situation observed which could be explained in terms of a high-order Mode Coupling transition.Peer reviewe
A classical molecular dynamics study of the diffusive motion of benzene molecules on graphite bas... more A classical molecular dynamics study of the diffusive motion of benzene molecules on graphite basal planes has been performed in the sub-monolayer coverage regime. Atomistic calculations were performed using the second generation forcefield COMPASS as well as the general purpose forcefields Dreiding and Universal. The COMPASS based calculations show evidence for a Brownian nature of the diffusion with a very small diffusion activation barrier of 11±2meV in agreement with recent helium and neutron spin-echo spectroscopy data [Fouquet P, Hedgeland H, Jardine AP, Alexandrowicz G, Allison W, Ellis J. Measurements of molecule diffusion on surfaces using neutron and helium spin echo. Physica B 2006;385–386:269-71]. Reasonable agreement is also found for the general purpose forcefields if screened charges are used in the description of the Coulombic non-bond interaction. The less computationally intensive Dreiding forcefield was shown to give a good qualitative description of the diffusion validating its applicability for future large scale calculation, i.e., for long times or enlarged systems. A potential energy surface (PES) has been established for the translational and rotational diffusive motion. The PES shows a peculiar dependence of the lateral diffusion barriers on the rotation angle of the benzene molecule which leads to a preferential selection of certain rotational states in the MD trajectories.
HAL (Le Centre pour la Communication Scientifique Directe), 2008
International audienceQuasi-elastic neutron scattering (QENS) measurements have been performed to... more International audienceQuasi-elastic neutron scattering (QENS) measurements have been performed to investigate the surface selfdiffusion of hydrogen molecules. A monolayer of molecular hydrogen was adsorbed on a carbon material commonly used in polymer electrolyte membrane fuel cells, called XC-72. QENS spectra were recorded at the time-of-flight spectrometer IN5 at Institut Laue-Langevin (ILL) in Grenoble at 40, 50, 60, and 70 K. By using the Chudley & Elliott model for jump diffusion, we found the diffusion coefficient at each temperature. The logarithm of the diffusion coefficient was plotted versus the inverse of the temperature to give the coefficient in the Arrhenius equation. From this, we can estimate the diffusion at higher temperatures as well. Our observed diffusion follows the same trend as that found for hydrogen molecules on the surface of singlewalled carbon nanotubes and Grafoil
The binding and structure of an ordered pyrazine overlayer on graphite are studied with neutron s... more The binding and structure of an ordered pyrazine overlayer on graphite are studied with neutron scattering and DFT. We find increased van der Waals bonding and stability of the overlayer compared to benzene, due to the nitrogen atoms in the ring.
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Papers by Peter Fouquet